U.S. Pat. No. 4,210,106 discloses a method of the kind referred to above wherein a control deviation is formed between the actual value of a variable indicating the lambda value of the engine exhaust and a pregiven fixed value; and, wherein an integration to form an actuating variable component takes place with the integration taking place in a direction of higher integration values (enriching) when a lean mixture is present and, when a rich mixture is present, a deviation takes place in the direction of lesser integration values (leaning). For a change of sign of the control deviation to that sign, which belongs to the desired shift of lambda mean value (this is referred to in the following as a stop signal), the integration is delayed for a time and for this purpose an integration-stop time span is used which is always then set anew when the present integration-stop time span has run.
For the purpose of explaining this procedure, it is assumed in the following embodiment and in other embodiments which follow that the lambda mean value should be shifted in the direction "rich" with respect to the lambda value "one". The following all applies to a shift in the lean direction when the terms "rich" and "lean" are exchanged with each other.
The magnitude with which the control deviation is formed is dependent upon the type of the probe used. If the probe supplies a signal which is directly proportional to the lambda value of the measured engine exhaust, then the control deviation is the difference between a pregiven lambda value, especially the value one, and the actual lambda value. If a probe having an intense non-linear relationship (jump response) between probe voltage and lambda voltage is used, the control deviation usually is formed as the difference between a pregiven voltage in the mean voltage range, for example, 450 mV, and the current probe voltage. The value of the voltage pregiven in the mean range is not all that critical since it is only important whether the probe signal just then shows lean or rich.
A lambda controller forms an output value with the aid of the control deviation with the output value usually being a multiplication factor for a precontrol injection time. This multiplication factor has approximately the value one when the precontrol injection time is so selected that this time matches quite precisely the injection time which is necessary for the particular operating state of the engine in order to adjust a lambda value close to one.
The output value, that is the control factor, has at least one integral component. In addition, or also exclusively, a fixed-value component can be used. In the following, the integral component is however decisive.
As long as a fixed component (often referred to as a proportional value) and/or an integral component are used at both sides of the lambda deviation zero, then precisely the lambda mean value one is obtained.
In accordance with U.S. Pat. No. 4,210,106, the procedure followed to shift the lambda mean value provides that fixed values always operate in the direction for obtaining a rich mixture and/or then, when the control deviation shows that after an upward integration (leading to a rich mixture), a downward integration would actually have to follow, the upward integration however is still maintained for a pregiven time duration. As an alternative to these possibilities, and this is not mentioned in this patent, the procedure can be followed that fixed values are added which are utilized in reference to the control deviation zero symmetrically for the formation of the output value which however show different amounts or that the integration value then when the control deviation actually requires a downward integration after an upward integration is no longer increased but rather for a pregiven time span is maintained unchanged. By varying the fixed-value amounts, the integration-stop time spans or also the integration speed, the extent of the mean value shift can be fixed. The parameter values used in each case can be permanently pregiven. However, the parameter values are preferably pregiven in dependence from the particular operating state of the engine at that time, as disclosed in U.S. Pat. No. 4,461,258.
FIG. 3 shows a lambda control of the kind referred to above wherein no disturbances are present for individual cylinders (FIG. 3A) whereas FIG. 4 shows the case of individual disturbances (FIG. 4A) in accordance with the mentioned method. Disturbances individual to a cylinder (often known as chemical noise) occur, for example, in that for a four-cylinder engine, the injection valve for one cylinder enriches the mixtures somewhat more than this applies for the mixture of the three other cylinders. This case is assumed in FIG. 4.
FIG. 3 shows that the control deviation changes the sign from minus to plus at a time point T1 (FIG. 3C) which shows that the mixture changes its composition from lean to rich. The output value FR (FIG. 3B) must act against the foregoing; however, as mentioned above, the obtained output variable value is retained at first for an integration-stop time span tv. After this time span has run, a fixed value jump pm takes place in the lean direction and integration takes place in the lean direction, that is, to a lesser control factor FR. As soon as the actual control factor drops below its neutral value (assumed in the example to be one) integration would actually have to take place in the opposite direction; however, it must be noted that the control factor acts on the injection time and therefore on the mixture composition at the inlet of the engine whereas the change caused thereby is only determined after a dead time td has run, namely at a time point T2 (FIG. 3C) by the oxygen probe mounted in the exhaust-gas flow. A fixed-value jump pf and an integration in the rich direction then takes place directly, that is, without maintaining an integration-stop time span. In this way, a rich mixture is finally again obtained which is determined by the probe delayed, namely at time point T3. This time point corresponds to time point T1 and, for this reason, the sequences described starting at this time point are repeated. Typically, the time span between the time points T1 and T3 is approximately 2 seconds. However, this time span is very dependent upon the distance of the gas outlet from the engine the oxygen probe which is mounted in the exhaust-gas flow and how high the flow velocity (determined by engine speed and load of the engine) of the exhaust gas is.
The above-mentioned neutral value of the control factor is the value at which every small deviation from the same effects a change of the mixture composition toward lean or rich depending upon the sign of the change. The time-dependent mean value FR of the control factor FR is influenced by the integration-stop time span tv. This mean value then no longer lies at the neutral value (here "one"); rather, for example, at the value 1.01. The mean value and therefore the lambda mean value can be so adjusted that the exhaust-gas composition falls into the permissible operating range of the catalytic converter used and therefore the toxic gas components are minimized.
From FIGS. 4B and 4C it is directly apparent that the above-mentioned sequence is greatly disturbed when disturbances according to FIG. 4A are present for individual cylinders. In the embodiment shown, the disturbances lead to an unwanted leaning of the time-dependent mean value FR of approximately 1% so that on average a control factor FR of approximately one is obtained even though with the aid of an integration-stop time span tv a control factor mean value of approximately 1.01 should actually be adjusted.
The task is then presented to provide a method and an arrangement for shifting the lambda mean value which can essentially ensure the desired shift even in the case of disturbances.